The UCSF group had earlier discovered a variant of the human Per2 gene that causes FASPS. They also showed that the so-called "S662G" variant, in which the serine building block normally present at position 662 is replaced by glycine, prevented a regulatory enzyme from tacking a phosphate onto the encoded protein.
Now, the researchers report additional evidence that the lost "phosphorylation" prevents a cascade of chemical modifications that are normally primed by the initial event.
Moreover, they show that the human gene inserted into otherwise normal mice causes them to rise early, symptoms that mirror those in people with FASPS. In contrast, a mutation that mimicked an increase in phosphorylation at amino acid 662 increased the transcription of PER2 and pushed the animals' sleep pattern later.
Their studies in mice revealed that the amino acid change associated with FASPS, which alters the charge of the residue, alters the ability of PER2 to regulate its own transcription. PER2 presumably manages such regulation through interaction with other proteins since it doesn't bind DNA itself, they said.
The findings led the researchers to suggest a model of clock function in which cells sense changing PER2 levels over time, beginning a new daily cycle when a certain threshold is crossed.
"In S662G individuals or mice, the alteration in transcription leads to production of less PER2, while the clock protein's degradation remains unaffected," they explained. Thus, the researchers added, "PER2 leve